Month: July 2018

It has been several weeks since my last post. Since that time, I adapted a real time signal viewer to display the messages transmitted by the wireless controller I programmed and installed to replace the existing PIC controller, which I carelessly blew up. The viewer displays 19 of the parameters transmitted by the controller. This real time or recorded view was helpful to figuring out what was going on, much better than examining huge message log files.

As shown on earlier entries on the wiki, the combustion chamber temperature rises upon ignition and gas, then reaches a steady state where it remains for a few minutes. The interior case temperature continues to rise during this time. When the interior temperature reaches about 40°C, the combustion temperature starts a sudden rise toward about 165°C, much hotter. However, before it gets there, combustion ceases for no apparent reason. As the temperature falls, it is still not possible to restart combustion using a combination of gas and igniter commands until the case temperature declines to about 30°C.

The programmed algorithm uses only the combustion temperature probe value. The case temperature and humidity is transmitted but not used for control. The controller is programmed with two combustion temperatures, a low and a high. The high temperature is intended as a safety feature, temporarily shutting the gas flow and cooling the chamber. However, I set the temperature to the highest value that would stop combustion before it failed. That value was 140°C. I introduced a new algorithm that, at that temperature, enters the startup state. The startup state turns off the gas and waits for the trap to cool down to the startup maximum temperature, now 40°C, then starts the trap.

This works during the hot summer days, but the unit oscillates on and off constantly until the outside temperature drops and the unit stays on for the evening and night until it gets hot the next day. The downside is that the trap is operating only about 50% of the time during the day, which is not very effective in catching the fewer mosquitos flying around in the heat. It also repeatedly cycles the igniter, consuming its useful life more rapidly.

Under the theory that the combustion stops because the gas flow is interrupted at high temperature, I installed a 40mm miniature 40mm 12 volt fan to blow fresh air from the switch opening onto the valve assembly. This fan has an output of 4.1 cfm, which seems laughable compared to the existing fan. Using sealing tape to create an air intake duct from the switch opening in the case, the fan was directed on the valve. There was no effect.

But it still had to be the gas valve. What else would stop combustion? I disassembled the valve, and cleaned the orifice and interior, which was not obviously clogged, and reassembled. No difference.

Today, I used a T9 bit to unscrew then remove the active part of the valve, and taped over the port with sealing tape to bypass it, using the tank valve instead (dangerous), and restarted. The trap worked perfectly! The combustion temperature rose to 130°C, and the interior temperature to 50°C, and stayed in a perfect steady state! This is definitive proof that the valve was causing both the temperature rise (runaway) and the stoppage, because neither occurred after bypassing the valve.

So either the coil was opening up, or the magnetic plunger was losing its magnetism at high temperature. I removed the coil, removed the contacts from the connector, inspected, cleaned, and measured the coil to be 200.4Ω, regardless of any vibration or wiggling. Both connectors had gold plated contacts in good condition. I sprayed DeOxit contact cleaner anyway. I took the plunger assembly from another valve, installed it into the Defender, and plugged the coil back into the controller, moving the connected in and out to dislodge any possible contamination on the male connector.

After one hour, the combustion probe temperature is a very steady 130°C, and the case temperature is a pretty hot 55°C. That is 15°C hotter than the previous failure point. Its working!

Changing two variables leaves the question: is it the coil / connector, or is it the valve mechanism that made the difference? After a day or so, I will restore the original mechanism and see if that works, and will know for sure.

The end is in sight. I hope I can find the screws to the case. Stay tuned.

The Defender stops combusting, cools down, and eventually stops with an error when running at a high temperature for a period of time. I have so far no explanation for this failure. Apparently, combustion just stops, even though the propane solenoid and air flow fan is still on. Should be impossible. A voltmeter on the valve solenoid indicated that the solenoid was continuously powered even when the temperature hit extremes, which refuted an earlier theory. Also, once it stops, it will  not restart until the trap completely cools down, regardless of the commands sent to the unit. The ambient case temperature at 40C is not that high to cause problems for the controller. It is possible that the conducted heat might mess up the solenoid.

We know that combustion requires fuel, oxygen, and, to start, ignition. Something must be interfering with the fuel or the air. Perhaps the gas valve is shutting down when its temperature (via heat conducted through the brass fittings) gets too high. Perhaps the air flow is closing down because of something swelling up and restricting the two passageways from the fan to the combustion chamber. I will have to disassemble to investigate (but not now). Fortunately, this problem occurs only during the heat of the day, when the sun is shining on the trap, and then it is too hot to catch mosquitos anyway. In the evening, when things have cooled down a bit, I can restart the trap. Actually, this failure saves propane and energy, but I would like the trap to run per spec.

Perhaps it would be wise to limit the temperature to, say 140C. But how to do this? The chamber temperature is a function of the gas, the air flow, and the ambient temperature.

We could reduce the gas flow by modulating the solenoid signal, e.g., one second off every X seconds. After the valve is shut, there is still sufficient gas to continue combustion for some period of time. Some experiments to see how long it takes closing to cause combustion to stop indicate it can tolerate several seconds of the propane solenoid off and still recover.

The air flow is a function of the restrictions from the net and ‘trumpet’ air intake. The installing the net box certainly reduces the air flow dramatically. Replacing the net box with a new one had no effect. We do not have the capability to increase the air flow, since the fan is on full blast, and we don’t want to fool with the vacuum apparatus.

Perhaps switching the gas off periodically would result in a lower operating temperature by causing a net pressure reduction and lower gas flow. But do we want to reduce the net gas flow? After all, we want the carbon dioxide to be emitted. … More later. For more information, see the wiki

If anyone can think of an explanation, please let us know.

When trying to troubleshoot the new wireless controller for my old Defender, I decided to leave no stone unturned and order a new catch basket (the gadget that works like a net to catch and hold the mosquitos) because the old one might be restricting the air flow too much because of accumulated mosquito awfulness on the screen material. At the last minute, I decided to throw in some Quick Clear cartridges so that I wouldn’t have to set up my electric air pump to blow into the unit every second tank. Well, lo and behold, the Mosquito Magnet brand cartridge is no longer available on the Mosquito Magnet web site (although other brands are still being sold). What was going on?

I called the Mosquito Magnet support line, waited a few minutes, and asked about the cartridges. The operator said there was a recall of units made between 2014 and 2016. She said that if the cartridge was used improperly, it would damage the regulator on those units, so the ‘product manager’ decided to withdraw the product. I asked whether it was necessary to periodically clean the trap to blow out the contaminants in the propane. She said this was no longer necessary.

She said the improper use was use while the trap was running,  which back-fed the high pressure gas into the exit port of the regulator, damaging it. Of course, using the air pump, I have done this several times. Fortunately, the regulator on my old units was not in the 2014-6 range, so presumably no damage was done.

Or was it? The pressure on the pump with the valve closed went up to 120 psi and stayed there. If it were blowing out contaminants, I would expect the pressure to drop as time went on, but no. Some times, the pressure would indicate around 40-50 psi, which I considered more reasonable. However, I now realize that this occurred when the solenoid was open, and I was back-feeding the regulator at that time. Anyway, the pump or cartridges never made much or any difference. And the directions were faulty: the cartridge would not run for 30 seconds — it was gone in one big whoosh. If partly unscrewed, a weak flow ran for about 30 seconds, but did that make any difference?

So, no more cartridges. I will keep it up with the pump, with the solenoid off (trap unplugged). Out of superstition or habit, I suppose. And I will change the nozzles or at least clean them from time to time.

This is not really a blog about repair, but rather the “Do It Yourself” method of keeping old Mosquito Magnets alive and catching Mosquitoes. By “yourself” I mean someone with a technical or engineering background. This does not contain the standard consumer oriented information, but rather information gained from sometimes hot and sweaty reverse engineering in the field, while constantly on guard for or attacked by uncaught mosquitoes.

By the way, we are spelling the plural of mosquito interchangeably as mosquitoes and mosquitos (which I prefer, but others do not). Reminds me of George Bush’s Sr’s  VP and the “potato” incident.

Since 2016, I have been maintaining a wiki on the MM Defender and Liberty. This wiki has contained dated sections that should have been in a blog. But it would have been too much work for a hobby project.

In July, 2018, a new project has started emerged to replace the existing controller in the Defender and Liberty with an inexpensive wireless development board. This has resulted in lots of entries into the project wiki that should be blog entries. This blog is created to hold these entries as separate posts.

There may seem like a lot of moaning and groaning in this blog. Sorry, but that is just the way it is. I really cannot afford to spend the time I do on this project, it is a bit of an embarrassment, given that I could buy a new trap for $300 or so. But it irks me to throw these machines away. I need about 3 of them, even though each one is supposed to cover about an acre more or less. I hate those mosquitos.